Fatty acids (lipids) are water-insoluble organic biomolecules that can be extracted from cells and tissues by nonpolar solvents such as chloroform, ether or benzene. Lipids have several important biological functions, serving as (1) structural components of membranes; (2) storage and transport forms of metabolic fuels; (3) a protective coating on the surface of many organisms; and, (4) cell-surface components concerned in cell recognition, species specificity and tissue immunity. More specifically, polyunsaturated fatty acids (PUFAs) are important components of the plasma membrane of the cell, where they may be found in such forms as phospholipids and also can be found in triglycerides. PUFAs also serve as precursors to other molecules of importance in human beings and animals, including the prostacyclins, leukotrienes and prostaglandins. There are two main families of PUFAs (i.e., the omega-3 fatty acids and the omega-6 fatty acids).
The human body is capable of producing most of the PUFAs which it requires to function. However, eicosapentaenoic acid (EPA; 20:5, delta-5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6, delta-4,7,10,13,16,19) cannot be synthesized efficiently by the human body and thus must be supplied through the diet. Since the human body cannot produce adequate quantities of these PUFAs, they are called essential fatty acids. Because of their important roles in human health and nutrition, EPA and DHA are the subject of much interest as discussed herein.
DHA is a fatty acid of the omega-3 series according to the location of the last double bond in the methyl end. It is synthesized via alternating steps of desaturation and elongation (see FIG. 15). Production of DHA is important because of its beneficial effect on human health. For example, increased intake of DHA has been shown to be beneficial or have a positive effect in inflammatory disorders (e.g., rheumatoid arthritis), Type II diabetes, hypertension, atherosclerosis, depression, myocardial infarction, thrombosis, some cancers and for prevention of the onset of degenerative disorders such as Alzheimer's disease. Currently the major sources of DHA are oils from fish and algae.
EPA and arachidonic acid (AA or ARA; 20:4, delta-5,8,11,14) are both delta-5 essential fatty acids. EPA belongs to the omega-3 series with five double bonds in the acyl chain, is found in marine food, and is abundant in oily fish from the North Atlantic. Beneficial or positive effects of increased intake of EPA have been shown in patients with coronary heart disease, high blood pressure, inflammatory disorders, lung and kidney diseases, Type II diabetes, obesity, ulcerative colitis, Crohn's disease, anorexia nervosa, burns, osteoarthritis, osteoporosis, attention deficit/hyperactivity disorder and early stages of colorectal cancer (see, for example, the review of McColl, J., NutraCos. 2(4):35-40 (2003)).
AA belongs to the omega-6 series with four double bonds. The lack of a double bond in the omega-3 position confers on AA different properties than those found in EPA. The eicosanoids produced from AA have strong inflammatory and platelet aggregating properties, whereas those derived from EPA have anti-inflammatory and anti-platelet aggregating properties. AA is recognized as the principal omega-6 fatty acid found in the human brain and an important component of breast milk and many infant formulas, based on its role in early neurological and visual development. AA can be obtained from some foods (such as meat, fish, and eggs), but the concentration is low.
Gamma-linolenic acid (GLA; 18:3, delta-6,9,12) is another essential fatty acid found in mammals. GLA is the metabolic intermediate for very long-chain omega-6 fatty acids and for various active molecules. In mammals, formation of long-chain PUFAs is rate-limited by delta-6 desaturation. Many physiological and pathological conditions such as aging, stress, diabetes, eczema, and some infections have been shown to depress the delta-6 desaturation step. In addition, GLA is readily catabolized from the oxidation and rapid cell division associated with certain disorders (e.g., cancer or inflammation).
As described above, research has shown that various omega fatty acids reduce the risk of heart disease, have a positive effect on children's development and on certain mental illnesses, autoimmune diseases and joint complaints. However, although there are many health benefits associated with a diet supplemented with these fatty acids, it is recognized that different PUFAs exert different physiological effects in the body (e.g., most notably, the opposing physiological effects of GLA and AA). Thus, production of oils using recombinant means is expected to have several advantages over production from natural sources. For example, recombinant organisms having preferred characteristics for oil production can be used, since the naturally occurring fatty acid profile of the host can be altered by the introduction of new biosynthetic pathways in the host and/or by the suppression of undesired pathways, thereby resulting in increased levels of production of desired PUFAs (or conjugated forms thereof) and decreased production of undesired PUFAs. Optionally, recombinant organisms can provide PUFAs in particular forms which may have specific uses; or, oil production can be manipulated such that the ratio of omega-3 to omega-6 fatty acids so produced is modified and/or a specific PUFA is produced without significant accumulation of other PUFA downstream or upstream products (e.g., production of oils comprising AA and lacking GLA).
The mechanism of PUFA synthesis frequently occurs via the delta-6 desaturation pathway. For example, long-chain PUFA synthesis in mammals proceeds predominantly by a delta-6 desaturation pathway, in which the first step is the delta-6 desaturation of linoleic acid (LA; 18:2, delta-9,12) and alpha-linolenic acid (ALA; 18:3, delta-9,12,15) to yield gamma-linolenic acid (GLA; 18:3, delta-6,9,12)) and stearidonic acid (STA; 18:4, delta-6,9,12,15), respectively. Further fatty acid elongation and desaturation steps give rise to arachidonic acid (AA or ARA) and eicosapentaenoic acid (EPA). Accordingly, genes encoding delta-6 desaturases, delta-6 elongase components (also identified as C18/20 elongases) and delta-5 desaturases have been cloned from a variety of organisms including higher plants, algae, mosses, fungi, nematodes and humans. Humans can synthesize long-chain PUFAs from the essential fatty acids, LA and ALA; however biosynthesis of long-chain PUFAs is somewhat limited (they are regulated by dietary and hormonal changes), and LA and ALA must be obtained from the diet.
Elongases which have been identified in the past differ in terms of the substrates upon which they act. They are present in both animals and plants. Those found in mammals can act upon saturated, monounsaturated and polyunsaturated fatty acids. However, those found in plants are specific for saturated and monounsaturated fatty acids. Thus, there is a need for a PUFA-specific elongase to produce polyunsaturated fatty acids (PUFAs) in plants.
The elongation process in plants involves a four-step process initiated by the crucial step of condensation of malonate and a fatty acid with release of a carbon dioxide molecule. The substrates in fatty acid elongation are CoA-thioesters. The condensation step is mediated by a 3-ketoacyl synthase, which is generally rate limiting to the overall cycle of four reactions and provides some substrate specificity. The product of one elongation cycle regenerates a fatty acid that has been extended by two carbon atoms (Browse et al., Trends in Biochemical Sciences 27(9):467-473 (September 2002); Napier, Trends in Plant Sciences 7(2): 51-54 (February 2002)).
WO 02/077213 (published Oct. 3, 2002) describes isolated nucleic acid molecules encoding a fatty acid elongase with specificity for linoleic acid or alpha-linolenic acid from Isochrysis galbana (i.e., delta-9 elongase).
U.S. Pat. No. 6,403,349 (issued to Mukerji et al. on Jun. 11, 2002) concerns the identification of nucleotide and amino acid sequences of an elongase gene derived from Mortierella alpina. 
WO 02/26946 (published Apr. 4, 2002) describes isolated nucleic acid molecules encoding FAD4, FAD5, FAD5-2 and FAD6 fatty acid desaturase family members which are expressed in long-chain PUFA-producing organisms, e.g., Thraustochytrium, Pythium irregulare, Schizichytrium and Crypthecodinium. It is indicated that constructs containing the desaturase genes can be used in any expression system including plants, animals, and microorganisms for the production of cells capable of producing long-chain PUFAs.
WO 98/55625 (published Dec. 19, 1998) describes the production of PUFAs by expression of polyketide-like synthesis genes in plants.
WO 98/46764 (published Oct. 22, 1998) describes compositions and methods for preparing long-chain fatty acids in plants, plant parts and plant cells which utilize nucleic acid sequences and constructs encoding fatty acid desaturases, including delta-5 desaturases, delta-6 desaturases and delta-12 desaturases.
U.S. Pat. No. 6,075,183 (issued to Knutzon et al. on Jun. 13, 2000) describes methods and compositions for synthesis of long-chain PUFAs in plants.
U.S. Pat. No. 6,459,018 (issued to Knutzon et al. on Oct. 1, 2002) describes a method for producing STA in plant seed utilizing a construct comprising a DNA sequence encoding a delta-6 desaturase.
Spychalla et al. (Proc. Natl. Acad. Sci. USA, 94:1142-1147 (1997)) describes the isolation and characterization of a cDNA from Caenorhabditis elegans that, when expressed in Arabidopsis, encodes a fatty acid desaturase which can catalyze the introduction of an omega-3 double bond into a range of 18- and 20-carbon fatty acids.
An alternate pathway for the biosynthesis of AA and EPA operates in some organisms (i.e., the delta-9 elongase/delta-8 desaturase pathway). Wherby LA and ALA are first elongated to eicosadienoic acid (EDA; 20:2, delta-11,14) and eicosatrienoic acid (EtrA; 20:3, delta-11,14,17), respectively, by a delta-9 elongase. Subsequent delta-8 and delta-5 desaturation of these products yields AA and EPA. The delta-8 pathway is present inter alia, in euglenoid species where it is the dominant pathway for formation of 20-carbon PUFAs.
WO 2000/34439 (published Jun. 15, 2000) discloses amino acid and nucleic acid sequences for delta-5 and delta-8 desaturase enzymes. Based on the information presented herein, it is apparent that the delta-8 nucleotide and amino acid sequences of WO 2000/34439 are not correct. However, the correct sequence is set forth in corresponding U.S. Pat. No. 6,825,017 (issued to Browse et al. on Nov. 30, 2004) that describes desaturases, in particular, delta-5 and delta-8 desaturases and their use in synthesizing PUFAs.
Applicants' Assignee's co-pending application having application Ser. No. 11/166,003 filed Jun. 24, 2005 discloses a delta-8 desaturase from Euglena gracilis. 
Wallis et al. (Arch. Biochem. and Biophys. 365(2):307-316 (May 1999)) describes the cloning of a gene that appears to encode a delta-8 desaturase in Euglena gracilis. This sequence appears to be the same sequence disclosed in WO 2000/34439.
Qi et al. (Nat. Biotech. 22(6):739-45 (2004)) describes the production of long-chain PUFAs using, among other things, a delta-8 desaturase from Euglena gracilis; however, the complete sequence of the delta-8 desaturase is not provided.
WO 2004/057001 (published Jul. 8, 2004) discloses amino acid and nucleic acid sequences for a delta-8 desaturase enzyme from Euglena gracilis. 
An expansive study of PUFAs from natural sources and from chemical synthesis are not sufficient for commercial needs. Therefore, it is of interest to find alternative means to allow production of commercial quantities of PUFAs. Biotechnology offers an attractive route for producing long-chain PUFAs in a safe, cost efficient manner in microorganisms and plants.
With respect to microorganisms, many algae, bacteria, molds and yeast can synthesize oils in the ordinary course of cellular metabolism. Thus, oil production involves cultivating the microorganism in a suitable culture medium to allow for oil synthesis, followed by separation of the microorganism from the fermentation medium and treatment for recovery of the intracellular oil. Attempts have been made to optimize production of fatty acids by fermentative means involving varying such parameters as microorganisms used, media and conditions that permit oil production. However, these efforts have proved largely unsuccessful in improving yield of oil or the ability to control the characteristics of the oil composition produced.
One class of microorganisms that has not been previously examined as a production platform for PUFAs (prior to work by the Applicants' Assignee), however, are the oleaginous yeasts. These organisms can accumulate oil up to 80% of their dry cell weight. The technology for growing oleaginous yeast with high oil content is well developed (for example, see EP 0 005 277B1; Ratledge, C., Prog. Ind. Microbiol. 16:119-206 (1982)), and may offer a cost advantage compared to commercial micro-algae fermentation for production of omega-3 or omega-6 PUFAs. Whole yeast cells may also represent a convenient way of encapsulating omega-3 or omega-6 PUFA-enriched oils for use in functional foods and animal feed supplements.
WO 2004/101757 and WO 2004/101753 (published Nov. 25, 2004) concern the production of PUFAs in oleaginous yeasts and are Applicants' Assignee's copending applications.
WO 2004/071467 (published Aug. 26, 2004) concerns the production of PUFAs in plants, while WO 2004/071178 (published Aug. 26, 2004) concerns annexin promoters and their use in expression of transgenes in plants; both are Applicants' Assignee's copending applications.
Applicants' Assignee's copending applications also include CL2698 (U.S. patent application Ser. No. 11/265,761, filed Nov. 2, 2005), CL3136 (U.S. patent application Ser. No. 11/264,784, filed Nov. 1, 2005) and CL3160 (U.S. patent application Ser. No. 11/264,737, filed Nov. 1, 2005) (methods of making EPA, ARA and DHA, respectively, in Yarrowia lipolytica), each claiming benefit of the earlier provisional filing date of CL2698 on Nov. 4, 2004.